In recent years, under social demands for energy conservation and resource conservation, the durability of a tire has been required, and thus rubber materials with excellent anti-fracture properties, abrasion resistance and anti-crack growth properties have been desired. Although natural rubber is known as rubber having excellent properties of those, due to soaring prices thereof it has been required to develop synthetic rubber having the durability similar to that of the natural rubber.
In order to make properties of the synthetic polyisoprene more similar to that of the natural rubber and thus to improve the durability, it has conventionally been performed to improve strain induced crystallization by synthetic polyisoprene having higher cis (For example, see Patent Documents 1 to 3). Although the durability of the synthetic polyisoprene has been thereby improved, since a large amount of catalyst is required to obtain a desired amount of the polyisoprene, there has been a problem that the synthetic polyisoprene contains a large amount of residual catalyst and thus the durability thereof under high severity conditions falls short of the durability of the natural rubber. Further, there has been a drawback that, in some technologies, use of halogen or aromatic solvents causes an extremely high environmental load during industrial production.
Also, it is known that, from polymers having an isoprene backbone, as compared with polymers made from other monomers, it is difficult to efficiently produce high molecular weight polymers. This is believed to be a factor of degradation of the durability under the high severity conditions.